The manual that came with my Kitchen Aid mixer (Accolade 400) clearly states, " Do not exceed Speed 2 when preparing yeast doughs as this may cause damage to the motor." But Daniel Leader in "Local Breads" calls for faster speeds, sometimes for rather extended times, in several formulas. For example, in the formula for Genzano Country Bread (pg.199), he says to mix at Speed "5 or 6 on a  KitchenAid mixer" for 10 minutes, then at Speed 10 for 8 to 10 minutes.